Advances have been made in the tracing of solar activity by radioactive sulfur isotopes

The Sun is the solar system’s main source of energy, controlling the Earth’s climate and hydrological system, thereby maintaining the activity and habitability of the Earth’s epigenetic environment. Reconstructing past solar activity histories is important for assessing the intensity and frequency of anomalous solar activity and predicting its impact on astronauts, modern scientific and technological communications, and ecosystems. High-energy cosmic rays bombarding the Earth’s atmosphere can produce radioactive isotopes (also known as cosmic nuclides, such as carbon-14, beryllium-10, etc.), which exist in multiple layers of the Earth system (such as the atmosphere, hydrosphere, cryosphere, biosphere, lithosphere, etc.). Since the intensity of solar activity can adjust the intensity of cosmic rays entering the Solar System, affecting the yield of cosmonuclides in the Earth’s atmosphere (pictured), it is possible to reconstruct past solar activity by measuring the content of cosnuclides in ice cores, tree rings, sediments and other carriers.

Some articles point out that with the rapid development of carbon-14 accelerated mass spectrometry analysis technology, high-resolution analysis can accurately identify solar activity anomalies (such as solar proton events) at the interannual scale, and even reconstruct planetary and astronomical extreme events such as changes in the Earth’s magnetic field and supernova explosions. Although the field is expected to flourish in the future, recent studies have found that the records of different cosmonuclides, different carriers, and different regions are inconsistent in the same period, resulting in a lot of controversy, and one of the reasons for the controversy is that the production, transformation process, and trans-cyclographic transmission and preservation process of cosmonuclides in the atmosphere are very complex, and it is difficult to accurately simulate the current model.

Lin Mang, researcher of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, found that radioactive sulfur isotopes (sulfur 35, half-life of about 87 days) may be one of the important cracking methods. The researchers integrated long-term observations of atmospheric sulfur sulfate 35 for solar cycle 24 (2008-2019) (Priyardashi et al., 2012; Lin et al., 2016, 2018) and found that the long-term variation of sulfur 35 coincided with the 11-year cycle of solar activity, proving that sulfur 35 was a faithful recorder of solar activity. Considering that 2015-2016 was the most intense and persistent El Niño event in the past century, the study further found that regional atmospheric circulation changes caused by this extreme event can alter sulfur-35 levels near the ground, and similar extreme climatic effects must be carefully considered when interpreting other cosmic nuclide records.

Schematic of solar activity affecting the isotope concentration of radioactive sulfur in the Earth’s atmosphere. When solar activity is intense (sunspots increase), the solar wind is stronger, and the galactic cosmic rays entering the solar system become weaker, so the production of cosmic nuclides becomes smaller.

In view of the impact of atmospheric sulfur-containing substances on acid rain, human health and climate change, the research of sulfur in atmospheric science is far richer and deeper than that of beryllium and other elements, and the transformation, transport and settlement process of sulfur 35 in the atmosphere can be simulated more accurately using atmospheric chemical transport models. The constraints of sulfur 35 observations are expected to improve the model’s simulation of atmospheric processes of cosmonuclelides such as beryllium 7, beryllium 10, and carbon 14. In addition, sulfur-35 is not considered in current cosmonucleus yield models, so sulfur-35 observational data can provide new constraints for validating these nuclear chemical models. The study suggests that scientists in nuclear chemistry, atmospheric science, marine science, geochemistry and other fields cooperate to build a unified multi-circle layer cosmic nuclide chemical transport model. The study is important for using cosmonuclides to accurately reconstruct past astronomical, geomagnetic and climatic events, predict the frequency and intensity of similar extreme events in the future, and assess their impact on ecosystems and human technology.

The results of the research were recently published online in PNAS under the title of Cosmic radiosulfur tracking of solar activity and the strong and long-lasting El Nino Events. The research work is supported by the National Natural Science Foundation of China Innovative Research Group Project, the “From 0 to 1” original innovation project of the Basic Frontier Scientific Research Program of the Chinese Academy of Sciences, and the major special team project of the Guangdong Provincial Laboratory of Southern Marine Science and Engineering. (Source: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences)

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